1. Field of the Invention
The present invention relates to a thermal conductivity sensor that detects the thermal conductivity of a sample gas by using a Wheatstone Bridge circuit and measures the concentration of a predetermined component in the sample gas based on the thermal conductivity.
2. Description of the Related Art
Conventionally, a thermal conductivity sensor of this kind is constructed in such a manner that one measurement resistor is assembled in a measurement cell; one reference resistor is assembled in a reference cell; and other resistors are disposed outside of the cells as external resistors, constituting a Wheatstone Bridge circuit, as described in Japanese Patent Application Laid-open (JP-A) No. 2003-42983.
However, since the disposal temperature and the temperature coefficient are different between the external resistors, and the measurement resistor and the reference resistor, the temperature influence received from the outside is different between the external resistors, and the measurement resistor and the reference resistor, thereby making it difficult to obtain correct measurement results.
For this reason, it is possible to consider providing a temperature correction circuit for correcting at least the temperature influence that the external resistors receive. However, the temperature influence that the external resistors receive may be different product by product due to the variation in the temperature coefficient of the external resistors, so that a correction coefficient of the temperature correction circuit must be set product by product, and the setting thereof is cumbersome, thereby causing an increase in costs.
Further, by electric signals from one measurement resistor and one reference resistor alone, the signal amount will be small, to aggravating the signal-to-noise (SN) ratio and, as a result thereof, the measurement in a low-concentration range is difficult.
There is also a thermal conductivity sensor based on the above-described type of sensor, in which one in which four cells (two measurement cells and two reference cells) are used, and a measurement resistor or a reference resistor is assembled in each cell; however, this raises problems in that the production costs will increase and scale reduction is difficult. Also, a reference gas such as an N2 gas must be enclosed in the two reference cells, thereby raising a problem that production errors, such as inevitable variation in the enclosure, lead to measurement errors.